This invention relates to a downhole tool, and in particular to a tool having radially extendable members, such as an underreamer or an expandable stabiliser.
In drilling operations, such as the drilling of bores to access subterranean hydrocarbon deposits, the drilled bores are lined with steel tubing, known as casing, which is cemented in place by pumping cement into an annulus between the casing and the bore wall. Once a length of casing is in place, this places restrictions on the diameter of any subsequent sections of bore, as drill bite and any further bore-lining casing must be able to pass through the existing casing. Clearly, reductions in bore diameter are undesirable as, for example, this limits the production flowrate of hydrocarbons through the bore.
It is known to underream beneath a section of casing to increase the bore diameter beyond the internal diameter of the casing in order to, for example, create a bore of sufficient diameter to allow a caning of outer diameter only slightly smaller than the inner diameter of the existing casing to be cemented in the underreamed bore. This is achieved by an underreaming tool which features radially extendable cutters which are initially retracted to permit the tool to be run into the bore on a drill string, and are then extended once the tool has passed beyond the end of the casing, so that drilling and underreaming may commence.
Underreamers of various forms are currently in use, though a number of existing tools have encountered some significant problems. For example, in certain situations underreamer blades have jammed in the extended configuration, such that the underreamer cannot be withdrawn through the existing casing. Of course, this prevents the drill string on which the underreamer is mounted from being withdrawn from the bore; rectifying this situation involves considerable inconvenience and expense.
It is among the objectives of embodiments of aspects of the present invention to obviate or mitigate this problem.
According to the present invention there is provided a downhole tool for mounting on a drill string, the tool comprising:
a mandrel;
a body axially movably mounted on the mandrel; and
a radially extendable member mounted in the body and being operatively associated with the mandrel such that relative axial movement of the mandrel and body induced radial movement of the member, application of axial tension to the tool producing axial movement of the body relative to the mandrel tending to retract the member from an extended configuration.
The tool may take the form of an underreamer or an expandable stabiliser. For brevity, reference will be made primarily to underreamers, in which the radially extendable member will he a cutter.
This aspect of the invention facilitates retraction of the member on removal of the tool from a bore through a restriction; if the member is extended when the tool encounters a bore restriction, the member will engage the restriction and by applying tension to the string the mandrel may be lifted relative to the body and member, or vice versa, thus allowing retraction of the member.
According to another aspect of the present invention there is provided a downhole tool for coupling to a drill string, the tool comprising:
a mandrel;
a body movably mounted on the mandrel; and
a radially extendable member mounted in the body and being operatively associated with the mandrel such that relative movement of the mandrel and the body induces radial movement of the member, the mandrel including means for drawing the member inwardly to permit positive retraction of the member from an extended configuration.
This aspect of the invention overcomes the difficulties associated with existing underreamers, in which the cutters may be extended by interaction between a cutter and a spring-loaded mandrel, the mandrel being movable to extend the cutter under the influence of fluid flow through the tool. In the event that the spring force, for whatever reason, is insufficient to retract the mandrel, the blades cannot be retracted.
Preferably, said means for drawing the member inwardly is in the form of an undercut groove on one of the mandrel and member and a corresponding profile on the other, which groove and profile may be of a T-slot or dovetail configuration.
According to a further aspect of the present invention there is provided a downhole tool for coupling to a drill string, the tool comprising:
a mandrel;
a body movably mounted on the mandrel and defining an channel;
a radially extendable member mounted in the body channel and being operatively associated with the mandrel such that relative movement of the mandrel and body induces radial movement of the member; and
the mandrel and the member including means for maintaining a desired orientation of the member relative to the body channel.
This aspect of the present invention avoids the difficulties associated with extendable members such as underreamer cutters xe2x80x9ccockingxe2x80x9d or jamming in body channels or apertures from application of axial or circumferential forces to the outer parts of the cutters producing a moment on the cutter and tending to rotate the cutter in the channel; it the cutter is maintained at a desired orientation to the body such rotation is avoided or at least minimised.
Preferably, said means for maintaining a desired orientation is in the form of a groove and cooperating profile, and moat preferably an undercut groove and corresponding profile, such as a dovetail configuration.
Preferably also, the body defines an aperture to accommodate the member.
According to a still further aspect of the present invention there is provided a downhole tool for mounting between first and second sections of a drill string, the tool comprising:
a mandrel for coupling to a first section of drill string;
a body axially movably mounted on the mandrel and for coupling to a second section of drill string whereby application of weight to the string induces axial movement of the body relative to the mandrel;
fluid pressure responsive means for moving the body axially relative to the mandrel on application of a fluid pressure force between the body and mandrel; and
a radially extendable member mounted in the body and being operatively associated with the mandrel such that relative axial movement of the mandrel and body induces radial movement of the member.
This aspect of the invention allows the member to be extended by one or both of weight applied to the string and by application of fluid pressure, for example by application of elevated pressure to the tool bore. Thus, using an underreamer in accordance with the invention, it is possible to ream in circumstances where an underreamer actuated only by applied weight will encounter difficulties; when drilling and reaming soft formations it may not be possible to apply sufficient weight to actuate the extendable cutters, and when xe2x80x9cre-reamingxe2x80x9d a section of bore it is not possible to apply weight to the string to compress an intermediate section of the string.
The first or second drill string section may take the form of a drill bit, that is the tool may be mounted on the end of a drill string and the drill bit mounted directly to the tool.
These various aspects of the present invention may be combined as desired, and a preferred embodiment of the present invention will incorporate all of the different aspects of the invention. Some preferred features of the various aspects of the invention are set out below.
Preferably, a plurality of members are provided, and most preferably the tool is provided with three members which retract and expand in unison.
Preferably also, the mandrel and body define a throughbore to permit drilling fluid to pass therethrough, and where the member is a cutter a conduit may extend from the throughbore to permit fluid to be directed towards the cutter. The conduit may be provided with a nozzle. Preferably, the throughbore is initially closed, and is opened by relative movement of the mandrel and body resulting in extension of the cutter. This prevents unnecessary loss of drilling fluid when the tool is not actuated, and the pressure drop when the conduit is opened provides an indication at surface that the cutter has been extended.
Preferably also, the mandrel and body are rotatably coupled, the coupling permitting axial movement. Most preferably, the coupling is formed of co-operating hexagonal parts, but may alternatively be defined by splined parts.
Preferably also, the mandrel defines a cam surface for co-operating with an inner face of the member. As noted above, in certain preferred aspects of the invention the faces of the mandrel and member positively engage, such that the member is positively retracted and the member orientation maintained constant as the member is extended and retracted.
Preferably also, the portion of the mandrel coupled to the drill string and the portion of the mandrel defining the cam surface are separable, and are coupled by cooperating screw threads. The portion of the mandrel coupled to the string is rotatably coupled to the body. On making up the mandrel portions, the screw thread is not fully made up, such that a degree of rotation is permitted between the mandrel portions. This prevents torque being transmitted from the string, through the mandrel to the cam surface and the member, which might otherwise result in misalignment of the can and member or misalignment between the cutter and body, and jamming of the member.
Preferably also, the mandrel and body are initially releasably fixed relative to one another, for example by means of one or more shear pins; this permits manipulation of the string without activation of the tool. In particular, where the tool is an underreamer and it is desired to drill and underreamer beneath an existing casing, it may first be necessary to drill through a casing shoe on the lower end of the casing; activation of an extendable cutter during such a drilling operation would result in damage to the existing casing. Accordingly, the casing shoe may be drilled out and the bore drilled beyond the end of the casing sufficiently to accommodate the underreamer before additional weight is applied to the string to shear the pin and activate the underreamer.
A further aspect of the invention, which may be provided separately or in combination with the other aspects described above, is the provision of one or more members, such as cutters or blades, mounted to a mandrel by dovetails or T-slots.
As used herein, the term drill string is intended to encompass any appropriate form of drill support, including drill pipe and coil tubing. Typically, the underreamers in accordance with the various aspects of the prevent invention will be mounted on the lower end of a drill string with the drill bit mounted directly to the lower end of the underreamer.